Photon counting device and photon counting method

ABSTRACT

A photon counting device includes a plurality of pixels each including a photoelectric conversion element configured to convert input light to charge, and an amplifier configured to amplify the charge converted by the photoelectric conversion element and convert the charge to a voltage, an A/D converter configured to convert the voltages output from the amplifiers of the plurality of pixels to digital values; and a conversion unit configured to convert the digital value output from the A/D converter to the number of photons by referring to reference data, for each of the plurality of pixels, and the reference data is created based on a gain and an offset value for each of the plurality of pixels.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.16/764,066, filed May 14, 2020, which is 371 of International PatentApplication No. PCT/JP2018/023144, filed Jun. 18, 2018, which claims thebenefit of priority to Japanese Patent Application No. 2017-225861,filed Nov. 24, 2017, the contents of each of which is incorporatedherein by reference.

TECHNICAL FIELD

The present disclosure relates to a photon counting device and a photoncounting method.

BACKGROUND ART

For example, Non-Patent Literature 1 describes a photon countingtechnique using a CMOS image sensor. In this technique, imaging isperformed under a condition that only one photon is incident on onepixel in one frame by increasing a frame rate of the image sensor.

CITATION LIST Patent Literature

[Non-Patent Literature 1] B Saleh Masoodian, Jiaju Ma, Dakota Starkey,Yuichiro Yamashita, and Eric R. Fossum, “A 1 Mjot 1040 fps 0.22e-rmsStacked BSI Quanta Image Sensor with Cluster-Parallel Readout”,Proceedings of 2017 International Image Sensor Workshop (IISW), May30-Jun. 2, 2017, P230-233

SUMMARY OF INVENTION Technical Problem

For example, when photon counting is performed using a CMOS imagesensor, it is conceivable to discriminate the number of photons on thebasis of a digital value output from an A/D converter. However, in theCMOS image sensor, each of pixels constituting the sensor has readoutnoise. Further, a gain and an offset value of a plurality of the pixelshave a variation in a certain range. Therefore, there is concern that,for example, a digital value when one photon is incident and a digitalvalue when two photons are incident may show the same value, and photoncounting accuracy may be degraded.

An object of an aspect of the present disclosure is to provide a photoncounting device and a photon counting method capable of curbingdegradation of photon counting accuracy.

Solution to Problem

A photon counting device according to an aspect includes a plurality ofpixels each including a photoelectric conversion element configured toconvert input light to charge, and an amplifier configured to amplifythe charge converted by the photoelectric conversion element and convertthe charge to a voltage; an A/D converter configured to convert thevoltages output from the amplifiers of the plurality of pixels todigital values; and a conversion unit configured to convert the digitalvalue output from the A/D converter to the number of photons byreferring to reference data, for each of the plurality of pixels,wherein the reference data is created on the basis of a gain and anoffset value for each of the plurality of pixels.

In such a photon counting device, the voltage according to the photoninput to the photoelectric conversion element is output from theamplifier. The output voltage is converted to a digital value by the A/Dconverter. The digital value is converted to the number of photons bythe conversion unit. The conversion unit converts the digital value tothe number of photons by referring to the reference data. The referencedata is created on the basis of the respective gains and offset valuesin the plurality of pixels. Therefore, even when there is a variation inthe gain and the offset value between the pixels, the conversion unitcan convert the digital value to the number of photons while curbing aninfluence of the variation. Therefore, degradation of photon countingaccuracy can be curbed.

Further, the reference data may include a plurality of pieces ofthreshold value data created on the basis of the gains and the offsetvalues for the plurality of pixels, and the conversion unit may convertthe digital value for each of the plurality of pixels to the number ofphotons on the basis of the plurality of pieces of threshold value data.With this configuration, even when there is variation in the gain andoffset value for each pixel, the number of photons can be derived usinga threshold value according to the variation.

Further, the reference data may include a parameter corresponding to thegain and the offset value, the parameter being a preset parameter commonto the plurality of pixels, and the conversion unit may correct thedigital value for each of the plurality of pixels on the basis of adeviation between the gain and the offset value, and the parameter, andconvert the corrected digital value to the number of photons. In thisconfiguration, since the digital value can be corrected according to avariation in the gain and the offset value, for example, the digitalvalue can be converted to the number of photons using the same thresholdvalue in all the pixels.

Further, the readout noise of the amplifier may be equal to or less than0.2 [e-rms]. In this case, for example, an incorrect detection rate canbe curbed to 1% or less. Further, the readout noise of the amplifier maybe equal to or less than 0.15 [e-rms]. In this case, for example, theincorrect detection rate can be curbed to 0.1% or less.

Further, the gain may be equal to or more than 10 [DN/e]. By increasingthe gain, it is possible to accurately reproduce an analog value that isoutput from the amplifier.

Further, a photon counting method of an aspect includes converting lightinput to respective photoelectric conversion elements constituting aplurality of pixels to charge; amplifying, by an amplifier constitutingthe plurality of pixels, the converted charge and converting the chargeto a voltage; converting, by an A/D converter, the voltages output fromthe respective amplifiers to digital values and outputting the digitalvalues; and converting the digital value of each of the plurality ofpixels output from the A/D converter to the number of photons on thebasis of respective gain and offset values of the plurality of pixels.

In such a photon counting method, the voltage output from the amplifieris converted to a digital value by the A/D converter according to inputphotons. The digital value is converted to the number of photons on thebasis of the respective gain and offset values of the plurality ofpixels. Therefore, even when there is a variation in the gain and theoffset value between the pixels, the digital value can be converted tothe number of photons while curbing an influence of the variation.Therefore, degradation of photon counting accuracy can be curbed.

Further, the converting of the digital value to the number of photonsincludes converting the digital value to the number of photons usingthreshold value data created for each of the plurality of pixels on thebasis of the respective gains and offset values in the plurality ofpixels. With this configuration, even when there is variation in thegain and offset value for each pixel, the number of photons can bederived using a threshold value according to the variation.

Further, the converting of the digital value to the number of photonsincludes correcting the digital value of each of the plurality of pixelson the basis of a deviation between the gain and the offset value, and aparameter, and converting the corrected digital value to the number ofphotons, and the parameter corresponds to the gain and the offset value,and is set in advance to be common to the plurality of pixels. In thisconfiguration, since the digital value can be corrected according to avariation in the gain and the offset value, for example, the digitalvalue can be converted to the number of photons using the same thresholdvalue in all the pixels.

Advantageous Effects of Invention

With the photon counting device and the photon counting method accordingto an aspect, it is possible to curb degradation of photon countingaccuracy.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram illustrating a configuration of a photon countingdevice according to an embodiment.

FIG. 2 is a graph showing a relationship between the number of electronsand a probability density.

FIG. 3 is a graph showing a relationship between readout noise and anincorrect detection rate.

FIG. 4A is a graph showing a relationship between the number ofelectrons and a probability density.

FIG. 4B is a graph showing a result of a simulation based on FIG. 4A.

FIG. 5A is a graph showing a relationship between the number ofelectrons and a probability density.

FIG. 5B is a graph in which a result of a simulation based on FIG. 5Ahas been converted to a digital value.

FIG. 6A is a graph showing a relationship between the number ofelectrons and a probability density.

FIG. 6B is a graph in which a result of a simulation based on FIG. 6Ahas been converted to a digital value.

FIG. 7A is a graph showing a relationship between the number ofelectrons and a probability density.

FIG. 7B is a graph in which a result of a simulation based on FIG. 7Ahas been converted to a digital value.

FIG. 8 is a diagram schematically illustrating a process of converting ameasured digital value to a number of photons.

FIG. 9 is a diagram schematically illustrating a process of converting ameasured digital value to a number of photons.

FIG. 10 is a diagram schematically illustrating a process of deriving anoffset value.

FIG. 11 is a diagram schematically illustrating a process of deriving again.

FIG. 12 is a diagram illustrating a correspondence between a gain and anoffset value, and a threshold value.

FIG. 13 is a diagram schematically illustrating a process of convertinga measured digital value to a number of photons.

FIG. 14 is a flowchart illustrating an operation of the photon countingdevice according to an embodiment.

FIG. 15 is a diagram illustrating a correspondence between a measureddigital value and a digital value after correction.

FIG. 16 is a flowchart illustrating an operation of a photon countingdevice according to another embodiment.

DESCRIPTION OF EMBODIMENTS

Hereinafter, embodiments will be specifically described with referenceto the drawings. For convenience, substantially the same elements aredenoted by the same reference numerals, and a description thereof may beomitted. Further, photon counting in the embodiments includes bothcounting of the number of photoelectrons generated in each pixel of animage sensor and counting of the number of photons in consideration of aquantum efficiency (QE) of an image sensor.

First Embodiment

FIG. 1 is a diagram illustrating a configuration of a photon countingdevice. As illustrated in FIG. 1 , the photon counting device 1 includesa CMOS image sensor 10, and a computer 20 connected to the CMOS imagesensor 10. The CMOS image sensor 10 includes a plurality of pixels 11and A/D converters 15. The plurality of pixels 11 are disposedtwo-dimensionally and arranged in a row direction and a columndirection. Each pixel 11 has a photodiode (a photoelectric conversionelement) 12 and an amplifier 13. The photodiode 12 accumulates electrons(photoelectrons) generated due to input of photons as charge. Theamplifier 13 converts the charge accumulated in the photodiode 12 to avoltage and amplifies the voltage. The amplified voltage is transferredto a vertical signal line 16 for each line (for each row) by switchingof a selection switch 14 of each pixel 11. A correlated double sampling(CDS) circuit 17 is disposed in each vertical signal line 16. The CDScircuit 17 removes noise that varies between pixels, and temporarilystores the transferred voltage.

The A/D converter 15 converts the voltage output from each of theamplifiers 13 in the plurality of pixels 11 to a digital value. In thefirst embodiment, the A/D converter 15 converts the voltage stored inthe CDS circuit 17 to a digital value. The respective digital valuesafter the conversion are output to the computer 20. For example, thedigital value may be sent to a horizontal signal line (not illustrated)by switching for column selection and output to the computer 20. Thus,in the CMOS image sensor 10, when photons are input to each pixel 11, adigital value according to the number of input photons is output to thecomputer 20. The A/D converter 15 may be provided in each pixel 11.

When the voltage amplified by the amplifier 13 is read, readout noise,which is random noise, is generated in the amplifier 13. FIG. 2 is agraph showing a probability distribution of electrons, in which ahorizontal axis represents the number of electrons and a vertical axisrepresents a probability density. The number of electrons generated bythe input photons follows a Poisson distribution, as illustrated in FIG.2 . In FIG. 2 , the probability distribution of electrons when twophotons are input to one pixel on average is shown for each readoutnoise.

Examples of the readout noise include 0.12, 0.15, 0.25, 0.35, 0.40,0.45, and 1.0 [e-rms]. When the readout noise is smaller, a peak of awaveform of the probability distribution appears to be sharper andseparation of distributions for respective numbers of electrons becomesclear, as illustrated in FIG. 2 . On the other hand, when the readoutnoise increases, distributions overlap with each other in the adjacentnumbers of electrons, and it becomes difficult to separate thedistributions of the respective numbers of electrons. For example, whenthe readout noise is equal to or less than 0.40 [e-rms], a peak for eachnumber of electrons appears to be identifiable. On the other hand, whenthe readout noise is equal to or more than 0.45 [e-rms], it is difficultto identify the peak for each number of electrons. In the firstembodiment, a magnitude of the readout noise allowing distinguishmentbetween the numbers of electrons to be distinguished has been obtainedaccording to whether or not the peak can be identified. Thereby, in theCMOS image sensor 10 of the first embodiment, the readout noise is equalto or less than 0.4 [e-rms]. An inflection point may be detected bysecondarily differentiating the probability distribution and themagnitude of the readout noise allowing distinguishment between thenumbers of electrons may be obtained.

When a threshold value for distinguishing between the adjacent numbersof electrons has been set, an incorrect detection rate of the number ofdetected electrons changes according to readout noise. FIG. 3 is a graphshowing a relationship between the readout noise and the incorrectdetection rate when a threshold value is an intermediate value betweenthe numbers of electrons, such as 0.5e, 1.5e, 2.5e, . . . . Theincorrect detection rate is a rate at which an incorrect number ofelectrons is detected, and is caused by the spread of the probabilitydistribution of the electrons. As illustrated in FIG. 3 , when theincorrect detection rate is desired to be equal to or less than 1%, thereadout noise needs to be equal to or less than 0.2 [e-rms]. Further,when the incorrect detection rate is desired to equal to or less than0.1%, the readout noise needs to be equal to or less than 0.15 [e-rms].

FIG. 4A is a graph showing a relationship between the number ofelectrons and the probability density. FIG. 4B is a graph showing aresult of a simulation based on FIG. 4A. In FIG. 4A, a probabilitydistribution of electrons when two photons are input to one pixel onaverage in a case in which readout noise is 0.15 [e-rms] is shown. InFIG. 4B, a distribution of the number of electrons for each number ofmeasurements is shown by an analog value. The A/D converter 15 convertsthe analog value illustrated in FIG. 4B to a digital value and outputsthe digital value. The digital value output from each pixel 11 is shownby the following equation.Digital value [DN]=gain [DN/e]×number of electrons [e]+offset value [DN]

FIGS. 5B, 6B, and 7B show graphs when the analog value of FIG. 4B hasbeen converted to a digital value. In FIGS. 5A, 6A, and 7A, aprobability distribution of electrons when two photons are input to onepixel on average in a case in which readout noise is 0.15 [e-rms] isshown, as in FIG. 4A. In FIGS. 5 to 7 , threshold values fordistinguishing the numbers of electrons are set with reference tointermediate values between the numbers of electrons, such as 0.5e,1.5e, 2.5e . . . . In FIGS. 5 to 7 , the threshold values are indicatedby broken lines. In FIG. 5B, the gain is 2 [DN/e] and the offset valueis 100 [DN]. As illustrated in FIG. 5B, when the gain is 2 [DN/e], it isdifficult for a variation in a measured value observed in an analogvalue to be reflected on the graph. Further, a proportion of output ofdigital values showing the same value as the threshold value is high.

In FIG. 6B, the gain is 10 [DN/e] and the offset value is 100 [DN]. Asillustrated in FIG. 6B, when the gain is 10 [DN/e], a distribution ofdigital values approximates to a distribution of analog values. On theother hand, since the gain is an even number, a digital valuecorresponding to the threshold value may be taken as illustrated in FIG.6B. In FIG. 7B, the gain is 11 [DN/e] and the offset value is 100 [DN].As illustrated in FIG. 7B, when the gain is 11 [DN/e], the distributionof the digital values approximates to the analog values.

Further, since the gain is an odd number, taking a digital valuecorresponding to the threshold value is curbed. Thus, by increasing avalue of the gain, the output digital value can further approximate tothe analog value. In the first embodiment, the CMOS image sensor 10 mayhave, for example, a gain equal to or more than 10 [DN/e].

Refer back to FIG. 1 . The computer 20 physically includes, for example,a storage device such as a RAM and a ROM, a processor (an arithmeticcircuit) such as a CPU, and a communication interface. Examples of sucha computer 20 include a personal computer, a cloud server, a smartdevice (a smartphone, a tablet terminal, or the like), a microcomputer,and a field-programmable gate array (FPGA). The computer 20 functions asa storage unit 21, a conversion unit 22, a data processing unit 23, anda control unit 24 by a CPU of a computer system executing a programstored in a storage device. The computer 20 may be disposed inside acamera including the CMOS image sensor 10, or may be disposed outsidethe camera. A display device 25 and an input device 26 can be connectedto the computer 20. The display device 25 is a display that can display,for example, a photon counting result obtained by the computer 20. Theinput device 26 is a keyboard, a mouse, and the like allowing a user toinput measurement conditions. A common touch screen may be used as thedisplay device 25 and the input device 26.

The storage unit 21 stores a table (reference data) for converting thedigital value output from the CMOS image sensor 10 to the number ofphotons. The table is, for example, a look-up table. The table iscreated on the basis of the respective gain and offset values of theplurality of pixels 11. In the first embodiment, threshold value dataassociated with each of the pixels 11 is stored as a table.

The conversion unit 22 converts the digital value of each of theplurality of pixels 11 output from the A/D converter 15 to the number ofphotons by referring to the table stored in the storage unit 21. Thedata processing unit 23 creates a two-dimensional image indicating thenumber of photons in each pixel 11 on the basis of the number of photonsoutput from the conversion unit 22. Further, the data processing unit 23may create, for example, a histogram, which is a plot of the number ofpixels with respect to the number of photons. The createdtwo-dimensional image or the like can be output to the display device25. The control unit 24 can generally control each function of thecomputer 20 or the CMOS image sensor 10.

Next, details of the photon counting device 1 will be described focusingon a process of the conversion unit 22. Hereinafter, for the sake ofsimplicity of a description, a description will be given assuming thatthe CMOS image sensor 10 of the photon counting device includes thepixels 11 arranged in 3 rows×3 columns.

First, a method of converting a digital value to a number of photonswhen it is assumed that there is no variation in gain and offset valueswill be described. FIG. 8 schematically illustrates a process ofconverting the measured digital value to the number of electrons. In theexample of FIG. 8 , it is assumed that the offset value is 100 [DN] andthe gain is 11 [DN/e] in each pixel. Further, the readout noise isassumed to be 0.15 [e-rms].

As illustrated in FIG. 8 , in such a CMOS image sensor 10, when photonsare input to each pixel 11, charge is accumulated in each pixel 11according to the number of photons. In the illustrated example, fiveelectrons are accumulated in all the pixels 11. The accumulated chargeis converted to a voltage by the amplifier 13 and is converted to adigital value by the A/D converter 15. In FIG. 8 , the digital value ineach pixel is shown inside the pixel. The converted digital value isconverted to the number of electrons. In this case, the digital value isconverted to the number of electrons using, for example, a thresholdvalue range. In the example of FIG. 8 , it is assumed that there is novariation in the gain and the offset value as described above.Therefore, when an upper limit and a lower limit of the threshold valuerange are intermediate values of the number of electrons, a thresholdvalue range corresponding to five electrons is equal to or more than4.5e and less than 5.5e. This threshold value range is 150 to 160 [DN]in a digital value. When the digital value illustrated in FIG. 8 hasbeen converted to the number of electrons by referring to this thresholdvalue range, the digital value is converted to five electrons in anypixel 11. Since the number of electrons generated by the input photonsfollows the Poisson distribution as described above, it is possible toobtain the average number of photons by dividing the average number ofelectrons for each pixel by the quantum efficiency. When the quantumefficiency is 100%, the number of electrons is the same as the number ofphotons.

Next, a case in which a digital value is converted to the number ofelectrons using the same threshold value range in a state in which thegain and the offset value have a variation will be considered. FIG. 9illustrates digital values when the gain and the offset value have avariation. In this example, an average gain is 11 [DN/e], and avariation σ in the gain is 10%. That is, a gain ±σ can have a value of9.9 to 12.1. Further, an average offset value is 100 [DN], and avariation σ in the offset value is 3%. That is, an offset value ±σ canhave a value from 97 to 103. In the example in FIG. 9 , a model in whichfive electrons are accumulated in all the pixels, as in FIG. 8 , isshown. When the same threshold value as in the example of FIG. 8 isused, four electrons are accumulated in the case of 139 to 149, fiveelectrons are accumulated in the case of 150 to 160, and six electronsare accumulated in the case of 161 to 171. Thus, it may be difficult toconvert the digital value to a correct number of electrons in a state inwhich the gain and the offset value have a variation.

Therefore, the conversion unit 22 of the first embodiment converts thedigital value for each of the plurality of pixels output from the A/Dconverter 15 to the number of electrons by referring to the table storedin the storage unit 21. The table has the threshold value data createdon the basis of the respective offset values and gains in the pluralityof pixels 11. FIG. 10 is a schematic diagram illustrating a process ofacquiring an offset value. The digital value is expressed by thefollowing equation as described above. Therefore, the offset value isindicated as a digital value output in a state in which no light isinput. Therefore, in the first embodiment, a plurality of digital valuesare acquired from a plurality of dark images acquired by the CMOS imagesensor 10 in a state in which light is not input, as illustrated in FIG.10 . The offset value is acquired by averaging the acquired digitalvalues for each pixel.Digital value [DN]=gain [DN/e]×number of electrons [e]+offset value [DN]

FIG. 11 is a schematic diagram illustrating a process of acquiring again. When a gain of each pixel is acquired, a plurality of frame imagesare acquired by the CMOS image sensor 10 with a sufficient amount oflight. An average optical signal value S [DN] of the digital value ineach pixel and a standard deviation N [DN] are acquired. Since the gainis expressed by N²/S, the gain is derived from the average opticalsignal value S and the standard deviation N.

The storage unit 21 of the first embodiment holds, as a table, thethreshold value data for each pixel derived on the basis of the gain andthe offset value. When the threshold value is an intermediate value ofthe number of electrons, a threshold value indicating a lower limit ofeach number of electrons and a threshold value indicating an upper limitthereof are respectively expressed by the following equations, and arange of the lower limit threshold value and the upper limit thresholdvalue is a threshold range corresponding to the number of electrons.Threshold value (lower limit)=(number of electrons−0.5)×gain+offsetvalueThreshold value (upper limit)=(number of electrons+0.5)×gain+offsetvalue

Thereby, for example, the threshold value when it is determined that thenumber of electrons is 5 electrons can be derived as illustrated in FIG.12 . FIG. 12 is a diagram illustrating a correspondence between the gainand the offset value, and the threshold value. In FIG. 12 , the acquiredgain and offset value is shown for each pixel. Further, a thresholdvalue when it is determined that the number of electrons is 5 is shownfor each pixel. For example, when the gain is 10.9 [DN/e]

and the offset value is 97.7 [DN], the lower limit threshold value is146.8 [DN] and the upper limit threshold value is 157.7 [DN]. Thestorage unit 21 has, as a table, threshold value data corresponding toeach number of electrons for each pixel. FIG. 13 is a diagramschematically illustrating a process of converting a measured digitalvalue of each pixel to the number of electrons. The conversion unit 22can derive a correct number of electrons from the digital value byreferring to the table stored in the storage unit 21. For example, inthe example of FIG. 9 , when the digital value is 162 [DN], it isdetermined that the number of electrons is 6, but in the example of FIG.13 , it is determined that the number of electrons is 5. The conversionunit 22 can obtain the average number of photons by dividing the averagenumber of electrons for each pixel by the quantum efficiency.

Next, an operation of the photon counting device 1 will be described.FIG. 14 is a flowchart illustrating an operation of the photon countingdevice. In the first embodiment, when the measurement is started in astate in which the photon counting device 1 is operated, photonsincident on the pixels 11 of the CMOS image sensor 10 are firstconverted to charge by the photodiodes 12 (step S11). The convertedcharge is converted to a voltage by the amplifier 13 (step S12). Thevoltage is converted to a digital value by the A/D converter 15 andoutput to the computer 20 (step S13). The digital value is compared witha threshold value set for each pixel 11 by the conversion unit 22 of thecomputer 20 (step S14), and is converted to the number of photons on thebasis of a comparison result (step S15). Thereby, the number of photonsinput to each pixel is measured. A measurement result may be displayedon the display device 25 as, for example, image data.

As described above, in the photon counting device 1, the digital valueis converted to the number of photons by the conversion unit 22. Theconversion unit 22 converts the digital value to the number of electronsusing a threshold value set for each pixel by referring to the tablestored in the storage unit 21. The table is created in consideration ofthe respective gains and offset values in the plurality of pixels 11.Therefore, even when there is a variation in the gain and the offsetvalue between the pixels 11, the conversion unit 22 can convert thedigital value to the number of electrons while curbing an influence ofthe variation. Therefore, degradation of photon counting accuracy can becurbed.

The table has a plurality of pieces of threshold value data respectivelycorresponding to the plurality of pixels 11. In this configuration,since appropriate threshold value data is created for each pixel 11according to variations in the gain and the offset value, the number ofelectrons can be accurately determined. Further, for example, it is notnecessary to correct the digital values in consideration of variations.

Readout noise of the amplifier 13 may be equal or less than 0.2 [e-rms].In this case, for example, the incorrect detection rate can be curbed to1% or less. Further, the readout noise of the amplifier 13 may be equalor less than 0.15 [e-rms]. In this case, for example, the incorrectdetection rate can be curbed to 0.1% or less.

The gain may be equal to or more than 10 [DN/e]. Since the CMOS imagesensor 10 has a high gain, it is possible to accurately reproduce ananalog value output from the amplifier 13 as a digital value.

Second Embodiment

A photon counting device according to a second embodiment is differentfrom the photon counting device of the first embodiment in aconfiguration of the storage unit 21 and the conversion unit 22.Hereinafter, differences from the first embodiment will be mainlydescribed. Since a configuration of the device is the same as that inthe first embodiment illustrated in FIG. 1 , a description thereof willbe omitted.

The storage unit 21 stores a table (reference data) for converting thedigital value output from the CMOS image sensor 10 to the number ofelectrons. The table is created on the basis of the respective gains andoffset values in the plurality of pixels 11. The storage unit 21 in thesecond embodiment stores a gain and an offset value for each pixel 11 asa table. Further, the storage unit stores the threshold value data foreach number of electrons common to all the pixels 11 as a table.

The conversion unit 22 may correct the digital value of each of theplurality of pixels 11 so that an influence of the variation in the gainand offset value among the plurality of pixels 11 is curbed, on thebasis of the respective gains and offset values in the plurality ofpixels 11, and convert the corrected digital value to the number ofphotons. In the second embodiment, the conversion unit 22 corrects thedigital value of each pixel 11 so that an apparent gain and offset value(parameter) of each pixel 11 become the same. The digital value afterthe correction can be derived on the basis of a deviation between a gainand an offset value of each pixel and an apparent gain and an apparentoffset value common to all the pixels. In this case, the digital valueafter the correction is derived using the following correction equation.The apparent gain and the apparent offset value may be set in advanceand stored in the storage unit 21 or the conversion unit 22.Digital value after correction=((digital value−offsetvalue)/gain)×apparent gain+apparent offset value

FIG. 15 is a diagram illustrating a correspondence between the measureddigital value and a digital value after correction. In an example ofFIG. 15 , an example in which the digital value measured by the CMOSimage sensor (see FIG. 9 ) 10 in the first embodiment has been correctedby the above correction equation is shown. In this example, theconversion unit 22 corrects the digital value so that the apparent gainin all the pixels is 11 [DN/e] and the apparent offset value is 100[DN]. That is, the digital value after the correction is derived usingthe following correction equation.Digital value after correction=((digital value−offsetvalue)/gain)×11+100

The conversion unit 22 acquires the number of electrons using thresholdvalue data common to the respective pixels with respect to the digitalvalue after the correction. For example, the storage unit 21 may hold athreshold value range derived by the following equation as a table. Theconversion unit 22 can convert the digital value after the correction tothe number of electrons by referring to the threshold value data held inthe table. In the example of FIG. 15 , since the apparent gain is 11[DN/e] and the apparent offset value is 100 [DN], it is determined thatthe number of electrons is 5 when the digital value after the correctionis 150 to 160. The conversion unit 22 can obtain the average number ofphotons by dividing the average number of electrons for each pixel bythe quantum efficiency.Threshold value (lower limit)=(number of electrons−0.5)×apparentgain+apparent offset valueThreshold value (upper limit)=(number of electrons+0.5)×apparentgain+apparent offset value

FIG. 16 is a flowchart illustrating an operation of the photon countingdevice. In the second embodiment, when the measurement is started in astate in which the photon counting device 1 is operated, light incidenton the pixels of the CMOS image sensor 10 is first converted to chargeby the photodiode 12 (step S21). The converted charge is converted to avoltage by the amplifier 13 (step S22). The voltage is converted to adigital value by the A/D converter 15 and output to the computer 20(step S23). The digital value is corrected for each pixel by theconversion unit 22 of the computer 20 (step S24). The corrected digitalvalue is compared with the set threshold value data (step S25), and isconverted to the number of photons on the basis of a comparison result(step S26). Thereby, the number of photons input to each pixel ismeasured.

In the second embodiment, the digital value is corrected according tothe variation in the gain and the offset value so that an influence ofthe variation is curbed. Therefore, the digital value after thecorrection can be converted to the number of electrons using the samethreshold value data in all pixels, as described above.

Although the embodiments have been described in detail with reference tothe drawings, a specific configuration is not limited to theembodiments.

For example, the conversion unit may obtain the number of electrons foreach pixel by dividing the value obtained by subtracting the offsetvalue from the measured digital value by the gain, as in the followingequation. In this case, a common threshold value range corresponding toall the pixels may be used for the obtained number of electrons. Forexample, a common threshold value range may be set with the upper andlower limits of the threshold value range being intermediate values ofthe number of electrons, as described above. In this case, the thresholdvalue range corresponding to five electrons is equal to or more than4.5e and less than 5.5e.Number of electrons=((digital value−offset value)/gain)

Further, in the CMOS image sensor 10 of the embodiment, an example inwhich the readout noise of each pixel is equal to or less than 0.4[e-rms] has been described. However, even when the readout noise is 0.4[e-rms] in a sensor specification, noise of some pixels may be greaterthan 0.4 [e-rms]. In such a case, pixels of which the readout noise isequal to or less than 0.4 [e-rms] may be ascertained in advance bymeasurement or the like, and photon counting may be executed using onlythe pixels of which the readout noise is equal to or less than 0.4[e-rms].

REFERENCE SIGNS LIST

1: Photon counting device

11: Pixel

12: Photodiode (photoelectric conversion element)

13: Amplifier

15: A/D converter

21: Storage unit

22: Conversion unit

The invention claimed is:
 1. A device for photon counting comprising: aCMOS image sensor including plurality of pixels each including aphotoelectric conversion element configured to convert input light tocharge, and an amplifier configured to amplify the charge converted bythe photoelectric conversion element and convert the charge to avoltage; an A/D converter configured to convert the voltages output fromthe amplifiers of the plurality of pixels to digital values; and acomputer configured to convert the digital value output from the A/Dconverter to a number of photons by referring to reference data, foreach of the plurality of pixels, wherein a readout noise of theamplifier is equal to or less than 0.4 e-rms, and wherein the referencedata is threshold value data for distinguishing between adjacent numbersof photons.
 2. The device according to claim 1, wherein the computer isconfigured to convert the digital value of only the pixels of which thereadout noise is equal to or less than 0.4 e-rms.
 3. The deviceaccording to claim 1, wherein the readout noise of the amplifier isequal to or less than 0.2 e-rms.
 4. The device according to claim 1,wherein the readout noise of the amplifier is equal to or less than 0.15e-rms.
 5. The device according to claim 1, wherein the computer isfurther configured to create a two-dimensional image indicating thenumber of photons in each pixel based on the number of photons.
 6. Thedevice according to claim 1, wherein the computer is further configuredto create a histogram that is a plot of the number of pixels withrespect to the number of photons.
 7. The device according to claim 1,wherein the A/D converter is provided in each pixel.
 8. The deviceaccording to claim 1, wherein the computer is configured to divide anaverage number of electrons for each pixel by a quantum efficiency toobtain an average number of photons.
 9. The device according to claim 1,wherein the reference data is a look-up table.
 10. A method for photoncounting the method comprising: converting light input to respectivephotoelectric conversion elements of a plurality of pixels of a CMOSimage sensor to charge; amplifying, by an amplifier constituting each ofthe plurality of pixels, the converted charge and converting the chargeto a voltage; converting, by an A/D converter, the voltage output fromthe amplifier to a digital value and outputting the digital value; andconverting the digital value of each of the plurality of pixels outputfrom the A/D converter to a number of photons by referring to referencedata, for each of the plurality of pixels, wherein a readout noise ofthe amplifier is equal to or less than 0.4 e-rms, and wherein thereference data is threshold value data for distinguishing betweenadjacent numbers of photons.
 11. The method according to claim 10,wherein the converting converts the digital value of only the pixels ofwhich the readout noise is equal to or less than 0.4 e-rms.
 12. Themethod according to claim 10, wherein the readout noise of the amplifieris equal to or less than 0.2 e-rms.
 13. The method according to claim10, wherein the readout noise of the amplifier is equal to or less than0.15 e-rms.
 14. The method according to claim 10, further comprising:creating a two-dimensional image indicating the number of photons ineach pixel based on the number of photons.
 15. The method according toclaim 10, further comprising: creating a histogram that is a plot of thenumber of pixels with respect to the number of photons.
 16. The methodaccording to claim 10, wherein the A/D converter is provided in eachpixel.
 17. The method according to claim 10, further comprising:dividing an average number of electrons for each pixel by a quantumefficiency to obtain an average number of photons.
 18. The methodaccording to claim 10, wherein the reference data is a look-up table.19. A computer comprising: a data processor configured to convert adigital value of each of the plurality of pixels output from an A/Dconverter of a CMOS image sensor to a number of photons by referring toreference data, for each of the plurality of pixels, wherein thereference data is threshold value data for distinguishing betweenadjacent numbers of photons.
 20. A non-transitory computer-readablestorage medium storing a program, the program causing the computer to:convert a digital value of each of the plurality of pixels output froman A/D converter of a CMOS image sensor to a number of photons byreferring to reference data, for each of the plurality of pixels,wherein the reference data is threshold value data for distinguishingbetween adjacent numbers of photons.